Investment bank UBS says that uncertainty around the new tax brackets could have resulted in some of the change over last year.

"Withholding tables for 2018 may have been adjusted incorrectly," the bank's economist Robert Martin said in a note to clients this week.

"As a result, nobody —not us, not other economists, not the US government —knows whether refunds will be larger or smaller than in past years, as small changes in the assumptions on any of the three factors lead to large changes in net payments to households."

It's not just refund size that's down, either. Every single cumulative metric tracked by the IRS is showing a negative percent change compared to last year. Total returns processed is down 25% while total returns received is down 12.4%.

UBS

The IRS assures that these are only early metrics and that operations are going smoothly despite the staff shortages right at the beginning of tax season due to the government shutdown.

"IRS systems are operating smoothly to start the filing season, and refunds are being issued," the agency said, according to Politico. "The IRS encourages taxpayers to e-file as the quickest way to receive their refunds."

That could all change come Friday, though, when funding runs out on February 15. As lawmakers work together to write a bipartisan border security proposal, a heavy cloud of uncertainty remains.